It is possible to measure the shear properties of polymeric liquids using conventional apparatus such as a cone and plate rotational rheometer but such measurements are of little relevance to behaviour under other flow conditions, especially extensional flow conditions which have the ability significantly to “stretch” macromolecules, providing orders of magnitude increases in elastic forces and extensional viscosity. Indeed, especially with dilute solutions of high molecular weight linear polymers, different flows can produce dramatically different rheological properties, in that such solutions may exhibit a reduction in viscosity under shear conditions but an increase under extensional flow conditions, under which the polymer macromolecules tend to become stretched out. Thus, a measurement of the rheological properties of such solutions under shear conditions is unlikely to provide an adequate indication of rheological performance under real flow conditions.
In order to achieve full chain extension of a polymer molecule so that the rheological properties of solutions thereof can be measured under extensional flow conditions, it is necessary for the flow field to provide an adequate stretching force and to maintain the force for a sufficient time. Generally, the strain in a given fluid element should be at least 100×, especially if the polymer molecules are long and highly flexible. In order to meet these criteria, it has hitherto been necessary to use a continuous pumping system applied to a minimum sample volume of about 200 ml, although typically a 500 ml sample has been used. However, for biological systems such sample volumes may not be obtainable and for polymer systems such volumes may be prohibitively expensive and the large amounts of organic solvents required for such sample volumes may represent a hazard.